Collaborative Research: SG: Fire, climate, and vegetation change in an oak savanna ecosystem from annual to millennial time scales

Fire and drought influence important ecosystem functions like productivity, carbon storage and nutrient cycling. Today, frequent fires are important for maintaining grasslands and savannas, which cover 40% of the surface of the Earth. Savannas are dynamic mixtures of trees and grass that are controlled by both fire and drought. It remains a mystery, however, exactly how the balance between trees and grasses is maintained over centuries or millennia. Furthermore, it is unclear how savanna ecosystems are likely to change over the long term and how changes may affect important ecological processes like carbon storage and nutrient cycling. This research will combine knowledge from a fire experiment begun in 1964, tree-ring records extending decades into the past, and a sediment core from a lake to understand how changes in drought and fire affect the oak savanna over time scales ranging from years to centuries. This research will provide both the first fire history of the oak savanna in the upper Midwestern U.S. and one of the only long-term nutrient records for this region. The resulting analyses will also provide information to managers in the Tallgrass Prairie and Oak Savanna Fire Science Consortium who need more information regarding long-term fire regimes and historical fire patterns to forecast future responses to environmental change. Many research products will be provided in open-access journals. The topic of savannas and fire will be shared with the public through the development of a video with the popular MinuteEarth Channel.

This research will integrate new paleoecological data with data from several decades of ecological experiments at Cedar Creek Ecosystem Science Reserve, a long-term research site in Minnesota, USA. These experiments, which are situated in a temperate oak savanna, suggest that increased fire frequency reduces the quantity of nitrogen and slows down its cycling both because of fire-induced nitrogen losses and because of a vegetation shift from an oak-dominated woodland to a grassland. Across a burn frequency gradient, soil samples below grass and trees will be measured for carbon, N concentration and isotopic composition. Tree-ring sampling of several species along the gradient will be used to develop new ring-width chronologies and stand age structures with precise dating and seasonal resolution. These chronologies will be used to assess growth response to N cycling changes, past drought episodes, and fire frequency. A Holocene-age sediment core from Cedar Bog Lake will be analyzed to assess fire history, past oak:grass dynamics, and N cycling metrics. With this novel combination of tree-ring data, lacustrine sediment measurements and information from a 53-year controlled burn experiment, researchers will address three research questions: (1) How have humans and climate influenced fire regime for the most recent 300 years compared with climate influences earlier in the Holocene? (2) How have fire and climate in turn interacted to influence oak dominance in mixed oak-grass vegetation? (3) How do changes in oak vs. grass dominance alter nitrogen cycling in the context of long-term changes in climate and fire frequency?